  // 节流
export function _throttle (fn, wait = 200) {
  let last, timer, now;
return function(){
    now = Date.now();
if(last && now - last < wait){
clearTimeout(timer);
      timer =setTimeout(function(){
        last = now;
fn.call(this,...arguments);
}, wait);
}else{
      last = now;
fn.call(this,...arguments);
}
};
}
